Fiber-optic temperature and flow sensor system and methods

ABSTRACT

A fiber optic sensor, a process for utilizing a fiber optic sensor, and a process for fabricating a fiber optic sensor are described, where a double-side-polished silicon pillar is attached to an optical fiber tip and forms a Fabry-Pérot cavity. In an implementation, a fiber optic sensor in accordance with an exemplary embodiment includes an optical fiber configured to be coupled to a light source and a spectrometer; and a single silicon layer or multiple silicon layers disposed on an end face of the optical fiber, where each of the silicon layer(s) defines a Fabry-Pérot interferometer, and where the sensor head reflects light from the light source to the spectrometer. In some implementations, the fiber optic sensor may include the light source coupled to the optical fiber; a spectrometer coupled to the optical fiber; and a controller coupled to the high speed spectrometer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit under 35 U.S.C. § 119(e) ofU.S. Provisional Application Ser. No. 62/161,730, filed May 14, 2015,and titled “FIBER-OPTIC TEMPERATURE AND FLOW SENSOR SYSTEM AND METHODS.”U.S. Provisional Application Ser. No. 62/161,730 is herein incorporatedby reference in its entirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under grantN00173-15-P-0376 awarded by the Naval Research Laboratory. TheGovernment has certain rights in this invention.

BACKGROUND

An optical fiber can include a flexible, transparent fiber made ofextruded glass (silica) or plastic. Light can be transmitted between twoends of the optical fiber, which may be used in fiber-opticcommunications. A fiber optic sensor uses an optical fiber either as thesensing element (e.g, an intrinsic sensor) or as a means of relayingsignals from a remote sensor to electronics that process a signal withinthe optical fiber (e.g., an extrinsic sensor). Fiber-optic sensors, suchas intrinsic sensors, utilize optical fibers to measure temperature,strain, pressure, and/or other characteristics associated with theoptical fiber.

SUMMARY

A fiber optic sensor, a process for utilizing a fiber optic sensor, anda process for fabricating a fiber optic sensor are described, where adouble-side-polished silicon pillar is attached to an optical fiber tipand forms a Fabry-Pérot (FP) cavity. In an implementation, a fiber opticsensor in accordance with an exemplary embodiment includes an opticalfiber configured to be coupled to a light source and a spectrometer; anda single silicon layer or multiple silicon layers disposed on an endface of the optical fiber, where each of the silicon layer(s) defines aFabry-Pérot interferometer, and where the sensor head reflects lightfrom the light source to the spectrometer. In some implementations,taking advantage of the dense interference fringes from the silicon FPcavity, average wavelength over multiple peaks or valleys or both isobtained as the signal to further improve the resolution. In someimplementations, the fiber optic sensor may include the light sourcecoupled to the optical fiber; a spectrometer coupled to the opticalfiber; and a controller coupled to the high speed spectrometer. In someimplementations, the fiber optic sensor may include a heating lightsource coupled to the optical fiber. The fiber optic sensor includes atleast one silicon layer and silicon attachment to the fiber for thepurpose of temperature and flow sensing. In an implementation, the flowcan include various flowing substances (e.g., water, liquid metal, air,other gasses, etc.) that provide cooling effects to the heated sensorhead.

In an implementation, a process for utilizing a fiber optic sensor inaccordance with an exemplary embodiment includes causing a light sourceto transmit light through an optical fiber and a circulator to a fiberoptic sensor, where the fiber optic sensor includes a silicon layerdisposed on an end face of the fiber optic sensor; receiving reflectedlight from the fiber optic sensor using a spectrometer, the spectrometerbased on a transmission grating and a diode array; and analyzing anoutput from the spectrometer based on received reflected light. In animplementation, the process for utilizing a fiber optic sensor includesa noise reduction implementation and/or a resolution enhancementimplementation based on an improved spectral shift calculation using theaverage wavelength of multiple peaks and/or valleys.

In an implementation, a process for fabricating a fiber optic sensor inaccordance with an exemplary embodiment includes forming at least onesilicon pillar on a silicon substrate; forming a thin film adhesive on aglass substrate; pressing an end face of an optical fiber onto the thinfilm adhesive; releasing the optical fiber with thin film adhesiveattached; placing the thin film adhesive on the end face onto the atleast one silicon pillar; and curing the thin film adhesive to providethe fiber optic sensor.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

DRAWINGS

The detailed description is described with reference to the accompanyingfigures. The use of the same reference numbers in different instances inthe description and the figures may indicate similar or identical items.

FIG. 1A is an isometric view illustrating an embodiment of a fiber opticsensor that includes a silicon layer disposed on an end face of anoptical fiber, in accordance with an example implementation of thepresent disclosure.

FIG. 1B is a partial side elevation cross section view illustrating anembodiment of a fiber optic sensor that includes multiple cascadedsilicon layers disposed on an end face of an optical fiber, inaccordance with an example implementation of the present disclosure.

FIG. 1C is a partial side elevation cross section view illustrating anembodiment of a fiber optic sensor that includes multiple cascadedsilicon layers disposed on an end face of an optical fiber, inaccordance with an example implementation of the present disclosure.

FIG. 1D is an environmental view illustrating an embodiment of a fiberoptic sensing system that includes a fiber optic sensor with a siliconlayer disposed on an end face of an optical fiber, in accordance with anexample implementation of the present disclosure.

FIG. 1E is an environmental view illustrating an embodiment of a fiberoptic sensing system that includes a fiber optic sensor with a siliconlayer disposed on an end face of an optical fiber, in accordance with anexample implementation of the present disclosure.

FIG. 1F is an environmental view illustrating an embodiment of acontroller used in a fiber optic sensing system, in accordance with anexample implementation of the present disclosure.

FIG. 1G is an isometric view illustrating an embodiment of a fiber opticsensor that includes a silicon layer disposed on an end face of anoptical fiber, in accordance with an example implementation of thepresent disclosure.

FIG. 1H is a graphical depiction illustrating a wavelength shift thatrepresents a temperature change using a fiber optic sensor that includesa silicon layer disposed on an end face of an optical fiber, inaccordance with an example implementation of the present disclosure.

FIG. 1I is a side elevation cross sectional view illustrating anembodiment of a fiber optic sensor that includes a silicon layerdisposed on an end face of an optical fiber, in accordance with anexample implementation of the present disclosure.

FIG. 2A is a flow diagram illustrating an example process for using afiber optic sensor that includes a silicon layer disposed on an end faceof an optical fiber, such as the fiber optic sensor illustrated in FIGS.1A through 1G.

FIG. 2B is a flow diagram illustrating an example process forfabricating a fiber optic sensor that includes a silicon layer disposedon an end face of an optical fiber, such as the fiber optic sensorillustrated in FIGS. 1A through 1G.

FIG. 3A is a diagrammatic partial cross-sectional side elevation viewillustrating the fabrication of a fiber optic sensor that includes asilicon layer disposed on an end face of an optical fiber, such as thefiber optic sensor illustrated in FIGS. 1A through 1G, in accordancewith the process shown in FIG. 2B.

FIG. 3B is a diagrammatic partial cross-sectional side elevation viewillustrating the fabrication of a fiber optic sensor that includes asilicon layer disposed on an end face of an optical fiber, such as thefiber optic sensor illustrated in FIGS. 1A through 1G, in accordancewith the process shown in FIG. 2B.

FIG. 3C is a diagrammatic partial cross-sectional side elevation viewillustrating the fabrication of a fiber optic sensor that includes asilicon layer disposed on an end face of an optical fiber, such as thefiber optic sensor illustrated in FIGS. 1A through 1G, in accordancewith the process shown in FIG. 2B.

FIG. 3D is a diagrammatic partial cross-sectional side elevation viewillustrating the fabrication of a fiber optic sensor that includes asilicon layer disposed on an end face of an optical fiber, such as thefiber optic sensor illustrated in FIGS. 1A through 1G, in accordancewith the process shown in FIG. 2B.

FIG. 3E is a diagrammatic partial cross-sectional side elevation viewillustrating the fabrication of a fiber optic sensor that includes asilicon layer disposed on an end face of an optical fiber, such as thefiber optic sensor illustrated in FIGS. 1A through 1G, in accordancewith the process shown in FIG. 2B.

DETAILED DESCRIPTION

Measurement of speed of gas or liquid flow is of great practicalimportance in a variety of industries, such as food inspection,pharmacy, oil/gas exploration, environmental, high-voltage powersystems, chemical plants, and oceanography research. Owing to their manyunique advantages, such as small size, light weight, immunity toelectromagnetic interference, remote sensing capability, harshenvironment tolerance, and capability for distributed orquasi-distributed measurement fiber-optic sensors, such as temperaturesensors, flowmeters, or anemometers, have proven to be attractivealternatives to their traditional mechanical or electromagneticcounterparts.

In addition to sensitivity and temperature range, two important sensorparameters include temperature resolution and speed (or response time).Temperature resolution, defined as the minimum detectable temperaturechanges, is determined by both the sensor sensitivity (defined as thesensor output from a given temperature change) and the noise of thesensor system, while the response time is mostly limited by the timeconstant of the heat transfer process between the sensing element andthe surrounding environment. The sensing element of many fiber-optictemperature sensors is part of the fiber itself, which is made of fusedsilica. The temperature resolution and the speed are limited by therelatively low thermo-optic coefficient (TOC) and thermal diffusivity ofthe glass material that lead, respectively, to a reduced sensorsensitivity and increased time for the temperature of the sensingelement to reach equilibrium with the surrounding environment. Forexample, it is well-known that a fiber Bragg grating (FBG), whosereflection spectrum features a single reflection peak, exhibit atemperature sensitivity of about 10 pm/° C. A fiber modal interferometerbased on a single mode-multimode-single mode fiber structure has beenreported to have a temperature resolution of 0.2° C. Manyall-silica-fiber-based temperature sensors possess relatively lowsensitivity and relatively low temperature resolution. As to theresponse time, a package of a FBG with a copper tube encapsulation cangreatly reduce the response time of the sensor from several seconds to48.6 milliseconds (ms) in water. A response time of 16 ms in air hasbeen demonstrated for a microfiber coupler tip temperature sensor.

Compared to fused silica, crystalline silicon is a much more desirablesensor material for high-resolution and high-speed temperature sensing.Silicon is highly transparent over the infrared wavelength and has a TOCapproximately 10 times larger than that of fused silica used for thesensing element for most fiber-optic sensors, resulting in potentiallymuch higher temperature sensitivity. In addition, a silicon-basedtemperature sensor also has high speed because of the large thermaldiffusivity of silicon, which is comparable to many metals (e.g.,aluminum and gold) and is more than 60 times larger than fused silica.However, the use of silicon as a temperature sensing element has notbeen utilized on a large scale for high-resolution and high-speedtemperature sensing. The dependence of the absorption of a silicon filmon temperature for temperature sensing and the sensor has shown arelatively low temperature resolution of ±0.12° C. and a long responsetime on the order of 1 second (s). A simpler structure with a thinsilicon film (thickness <1 μm) deposited directly on the fiber endthrough electron-beam evaporation has shown a temperature resolution ofonly 3° C. In this case, radio-frequency sputtering was applied tosimplify the deposition process, and the resolution was mainly limitedby the small thickness of the silicon film that led to broad spectralfringes. Instead of silicon film, a silicon micro-waveguide patterned ona micro-electro-mechanical system (MEMS) was developed as temperaturesensor, and due to the increased length of the Si sensing element, thetemperature resolution was improved to 0.064° C. However, it is achallenge to integrate the fiber and the MEMS into a single sensordevice, and the large size of previous sensing elements also limit theirtemperature measurement speed.

Accordingly, a fiber optic sensor, a process for utilizing a fiber opticsensor, and a process for fabricating a fiber optic sensor aredescribed, where a double-side-polished silicon pillar is attached to anoptical fiber tip and forms a Fabry-Pérot (FP) cavity and a sensor head.In an implementation, a fiber optic sensor in accordance with anexemplary embodiment includes an optical fiber configured to be coupledto a light source and a spectrometer; and a single silicon layer ormultiple silicon layers disposed on an end face of the optical fiber,where each of the silicon layer(s) defines a Fabry-Pérot interferometer,and where the sensor head reflects light from the light source to thespectrometer. In some implementations, the fiber optic sensor mayinclude the light source coupled to the optical fiber; a spectrometercoupled to the optical fiber; and a controller coupled to the high speedspectrometer. In some implementations, the fiber optic sensor mayinclude a heating light source coupled to the optical fiber.

In an implementation, a process for utilizing a fiber optic sensor inaccordance with an exemplary embodiment includes causing a light sourceto transmit light through an optical fiber and a circulator to a fiberoptic sensor, where the fiber optic sensor includes a silicon layerdisposed on an end face of the fiber optic sensor; receiving reflectedlight from the fiber optic sensor using a spectrometer, the spectrometerbased on a transmission grating and a diode array; and analyzing anoutput from the spectrometer based on received reflected light.

In an implementation, a process for fabricating a fiber optic sensor inaccordance with an exemplary embodiment includes forming at least onesilicon pillar on a silicon substrate; forming a thin film adhesive on aglass substrate; pressing an end face of an optical fiber onto the thinfilm adhesive; releasing the optical fiber with thin film adhesiveattached; placing the thin film adhesive on the end face onto the atleast one silicon pillar; and curing the thin film adhesive to providethe fiber optic sensor.

The high-speed fiber optic sensor and processes disclosed hereinincrease the temperature response and/or decrease the response time ofthe sensor. The diameter of the pillar (e.g., 80 μm or 100 μm, smallerthan the optical fiber diameter) leads to a fast temperature response.The length of the pillar (e.g., ˜200 μm) together with the largerefractive index (RI) of the silicon material result in dense fringes inthe reflection spectrum of the FP cavity. Using this unique spectralcharacteristic of the fiber optic sensor, the noise is significantlyreduced and the measurement resolution is improved. The sensor describedherein has, in one exemplary implementation, shown a high temperatureresolution of 6×10⁻⁴° C. and a short response time of 0.51 ms. Thisfiber optic sensor with a high-resolution and fast-response isespecially effective in the precise and real-time characterization oftemperature structure in highly dynamic optical turbulence.

Example Implementations

FIGS. 1A through 1I illustrate a fiber optic sensor 100 and fiber opticsensing system 130 in accordance with an example implementation of thepresent disclosure. The fiber optic sensor 100 can include an opticalfiber 102 configured to be coupled to a light source 126 and a highspeed spectrometer 128. The fiber optic sensor 100 and fiber opticsensing system 130 may be utilized as a temperature sensor indetermining temperature in gas and liquid.

In implementations, the optical fiber 102 can include a flexible,transparent fiber core 104 made of extruded glass (e.g., silica) or apolymer. The optical fiber 102 can be configured to transmit lightbetween the two ends of the optical fiber 102. In some instances, theoptical fiber 102 may be immune to electromagnetic interference.

The optical fiber 102 can include the core 104 and/or a cladding 106.The core 104 may include a fiber of glass and/or plastic that extendsalong the length of the optical fiber 102. The core 104 may besurrounded by a cladding 106, which may include a material with a lowerindex of refraction than the core 104. In embodiments, the cladding 106may include a cladding of a different glass and/or plastic, a bufferlayer, and/or a jacket.

As illustrated in FIG. 1A, the fiber optic sensor 100 can include asilicon layer 108 disposed on an end face 152 of the optical fiber 102(e.g., a cleaved portion of the optical fiber 102), which forms a sensorhead. In implementations, the silicon layer 108 can include a siliconpillar and/or a silicon-based film. In a specific embodiment, thesilicon layer 108 includes a double-sided polished silicon pillar. Inanother specific embodiment, the silicon layer 108 can include a pieceof a silicon wafer bonded to the end face 152. The silicon layer 108 mayinclude various diameters and/or lengths. For example, the silicon layer108 can include a silicon pillar with a diameter between about 80 μm to100 μm with a length of about 200 μm. In another example, the siliconlayer 108 may include a piece of silicon that is approximately 10 μmthick. In yet another example, the silicon layer 108 can include a pieceof silicon that is approximately 200 μm thick. It is contemplated thatthe silicon layer 108 may include other diameters and/or lengths. Thesilicon layer 108 diameter is generally less than the diameter of theoptical fiber 102, which leads to a fast temperature response. Thesilicon in the silicon layer 108 is highly transparent over the infraredwavelength and has a TOC approximately 10 times larger than that of thesilica used in the optical fiber 102 for most fiber-optic sensors,resulting in potentially much higher temperature sensitivity. Inaddition, the fiber optic sensor 100 also has high speed because of thelarge thermal diffusivity of silicon, which is comparable to many metals(e.g., aluminum and gold) and more than 60 times larger than fusedsilica. In other implementations, the silicon layer 108 may be replacedwith other materials that have large thermal diffusivity and highthermo-optic and thermal expansion coefficients. Further, it iscontemplated that the silicon layer 108 can include otherconfigurations, such as a cuboid configuration.

As illustrated in FIGS. 1B and 1C, the fiber optic sensor 100 mayinclude cascaded Fabry-Pérot cavities. In these implementations, thefiber optic sensor 100 can include an adhesive 156 disposed on an endface 152 of the optical fiber 102, a silicon layer 108 disposed on theadhesive 156, a second adhesive 166 disposed on the silicon layer 108,and a second silicon layer 168 disposed on the second adhesive 166. Thesilicon layer 108 and the second silicon layer 168 include the samematerial so that each has the same responsivity to temperature. In sucha way, the large free spectrum range (FSR) of the envelope originatingfrom the second silicon layer 168 (the second Fabry-Pérotinterferometer) provides large dynamic range, while the recognized densefringes with small FSR stemming from the silicon layer 108 (the firstFabry-Pérot interferometer) offers high resolution due to the narrowfringes. The adhesive 156 and/or the second adhesive 166 can includematerials suitable to bond the optical fiber 102, the silicon layer 108,and/or the second silicon layer 168 (e.g., a UV glue, etc.).Additionally, the adhesive 156 and/or the second adhesive 166 can be thesame or similar diameter as the silicon layer 108 and/or second siliconlayer 168, while the thickness of the adhesive 156 and/or the secondadhesive 166 can be only a few microns (e.g., <1 μm, 2 μm, 3 μm, etc.).Because the adhesive 156 and/or the second adhesive 166 are very thincompared to the silicon layer 108 and the second silicon layer 168, theyshow negligible influence on the reflection spectrum of reflected light162. In specific embodiments, the silicon layer 108 and the secondsilicon layer 168 can be the same or similar diameters but havedifferent lengths (e.g., the silicon layer 108 is 200 μm in length andthe second silicon layer 168 is 10 μm in length). It is contemplatedthat the silicon layer 108 and/or the second silicon layer 168 caninclude a variety of lengths and/or diameters. In FIG. 1C, n_(i) andD_(i) represent the refractive index and separation of the i^(th) layer,respectively. These implementations provide an optical fiber thermometerbased on double cascaded Fabry-Pérot interferometers both made from thesame material of silicon but with vastly different cavity lengths toachieve both large dynamic and high resolution.

In implementations, the fiber optic sensor 100 defines and includes aFabry-Pérot (FP) cavity. A Fabry-Pérot cavity (or Fabry-Pérotinterferometer) can include a cavity formed by the optical fiber 102 andthe silicon layer 108 disposed on the end face 152 of the optical fiber102. Due to the thermo-optic effect, temperature variations change theoptical thickness of the FP cavity and consequently cause spectralshifts in its reflection spectrum.

As illustrated in FIG. 1D, the fiber optic sensing system 130 caninclude the fiber optic sensor 100, a light source 126, a circulator124, a spectrometer 128, and a controller 132. In some implementations,the fiber optic sensing system 130 may include a heating light source146.

In implementations, the light source 126 (e.g., a broad band source)transmits light to the circulator 124 and the fiber optic sensor 100. Inone specific embodiment, light source 126 includes a wavelength sweptlaser, such as a high-speed, narrow-linewidth, and wavelength-sweptlaser. In another specific embodiment, light source 126 includes a laserdiode. In yet another specific embodiment, light source 126 includes awhite light source (e.g., 1550 nm). It is contemplated that the lightsource 126 can include other types of light sources. In implementations,the light source 126 is optically coupled to the optical fiber 102,which is optically coupled to a circulator 124. Additionally, the lightsource 126 can be coupled to and controlled using controller 132.

As illustrated in FIG. 1E, the fiber optic sensing system 130 mayinclude a heating light source 146 configured to provide heating light142. In these embodiments, the heating light source 146 can include alight source, such as a red laser diode, that is optically coupled tothe optical fiber 102 using a coupler 164. In one specific instance, theheating light source 146 can include a 635 nm diode laser. The heatinglight source 146 may include other light sources that provide light,which can be absorbed by the fiber optic sensor 100 and/or the siliconlayer 108. Additionally, the heating light source 146 can be controlledusing controller 132.

A circulator 124 can include a fiber-optic component used to separateoptical signals in optical fiber 102. In implementations, circulator 122can direct transmitted light 140 from light source 126 (and/or heatinglight 142 from heating light source 146) to fiber optic sensor 100 whiledirecting reflected light 162 from the fiber optic sensor 100 tospectrometer 128.

The fiber optic sensing system 130 can include a spectrometer 128coupled to the optical fiber 102 and a controller 132. Inimplementations, a spectrometer 128 can include a light sensor, such asa photodetector, configured to detect reflected light 162 and theassociated spectra from the optical fiber 102 and fiber optic sensor100. In a specific embodiment, the spectrometer 128 may include ahigh-speed photodetector (e.g., the high speed spectrometer from IbsenPhotonics, I-MON 256 USB, Denmark). Additionally, the spectrometer 128can be coupled to and controlled using controller 132.

As reflected light 162 is received and/or detected by spectrometer 128,a shift in wavelength is detected when temperature changes at thesilicon layer 108. The wavelength of the N^(th) fringe peak, λ_(N), ofthe reflection spectrum is given as

$\begin{matrix}{{\left( {N + \frac{1}{2}} \right)\lambda_{N}} = {2\;{nL}}} & {{Eq}.\mspace{14mu} 1}\end{matrix}$where n and L are, respectively, the RI and cavity length of the FPcavity. Both n and L are dependent on temperature due to thethermo-optic effect and the thermal expansion of the silicon material.Therefore, temperature change can be measured by monitoring λ_(N). FromEq. 1, the temperature sensitivity is given by

$\begin{matrix}{\frac{\partial\lambda_{N}}{\partial T} = {\lambda_{N}\left( {{\frac{1}{n}\frac{\partial n}{\partial T}} + {\frac{1}{L}\frac{\partial L}{\partial T}}} \right)}} & {{Eq}.\mspace{14mu} 2}\end{matrix}$Although Eqs. 1 and 2 only depict one of the multiple peaks in thereflected spectrum from the sensor, in some cases, an average wavelengthmay be applied to significantly reduce the noise lever or increase theresolution. This average wavelength can be obtained from multiple peaksor valleys or both.

The sensitivity depends on the TOC and the thermo-expansion coefficient(TEC) of the sensing material. The TOC and TEC for silicon are,respectively, 1.5×10⁻⁴ RIU/° C. and 2.55×10⁻⁶ m/(m·° C.) at 25° C. Toestimate the sensitivity, these values are applied to Eq. (2) at thepeak wavelength λ_(N) around 1550 nm and the RI of silicon is assumed tobe 3.4. From this, the sensitivity of the temperature sensor proposedhere is estimated to be 72 pm/° C. As a comparison to the all-fiberbased sensor, the TOC and TEC for fused silica are, respectively,1.28×10⁻⁵ RIU/° C. and 5.5×10⁻⁷ m/(m·° C.) at 25° C., both of which aremuch smaller than those for silicon. Assuming the RI of silica at 1550nm is 1.5, the sensitivity of an all-fiber based sensor is about 14 pm/°C., which is more than 5 times smaller than the fiber optic sensor 100.

The high RI (about 3.4) of silicon over infrared wavelength rangeproduces a high reflectivity at the interfaces between silicon layer 108and the surrounding environment and between silicon layer 108 and thefiber end face 152, which facilitates to obtain a large optical powerand a high fringe-visibility of the interferometric spectrum from the FPcavity for improving the sensor resolution. In addition, the high RI andthe relatively long FP cavity yield a large number of fringes within thewavelength range of the spectrometer, which can be exploited to furtherincrease the temperature resolution.

The fiber optic sensor 100 also features a short response time. Due tothe high thermal diffusivity of silicon and the small size of the sensorhead, the temperature within the FP cavity can quickly reach equilibriumwith surroundings.

As illustrated in FIG. 1B, the fiber optic sensing system 130 caninclude a controller 132 that is configured to determine a shift inspectra detected by spectrometer 128 using a fiber optic sensor 100. Thecontroller 132 can be coupled to the components of the fiber opticsensing system 130. Additionally, the controller 132 may be configuredin a variety of ways. As illustrated in FIG. 1F, the controller 132 isillustrated as including a processor 134, a memory 136, and acommunications interface 138. The processor 134 provides processingfunctionality for the fiber optic sensor 100 and may include any numberof processors, micro-controllers, or other processing systems, andresident or external memory for storing data and other informationaccessed or generated by the fiber optic sensor 100. The processor 134may execute one or more software programs that implement the techniquesand modules described herein. The processor 134 is not limited by thematerials from which it is formed or the processing mechanisms employedtherein and, as such, may be implemented via semiconductor(s) and/ortransistors (e.g., electronic integrated circuits (ICs)), and so forth.

The memory 136 is an example of a non-transitory computer storage devicethat provides storage functionality to store various data associatedwith the operation of the fiber optic sensor 100, such as the softwareprogram and code segments mentioned above, computer instructions, and/orother data to instruct the processor 134 and other elements of the fiberoptic sensor 100 to perform the techniques described herein. Although asingle memory 136 is shown, a wide variety of types and combinations ofmemory may be employed. The memory 136 may be integral with theprocessor 134, stand-alone memory, or a combination of both. The memorymay include, for example, removable and non-removable memory elementssuch as RAM, ROM, Flash (e.g., SD Card, mini-SD card, micro-SD Card),magnetic, optical, USB memory devices, and so forth.

The communications interface 138 is operatively configured tocommunicate with components of the fiber optic sensor 100. For example,the communications interface 138 can be configured to transmit data forstorage in the controller 132, retrieve data from storage in thecontroller 132, and so forth. The communications interface 138 is alsocommunicatively coupled with the processor 134 to facilitate datatransfer between components of the fiber optic sensing system 130 andthe processor 134 (e.g., for communicating inputs to the processor 134received from a device communicatively coupled with the fiber opticsensing system 130). It should be noted that while the communicationsinterface 138 is described as a component of fiber optic sensing system130, one or more components of the communications interface 138 can beimplemented as external components communicatively coupled to the fiberoptic sensing system 130 via a wired and/or wireless connection. Thefiber optic sensing system 130 can also comprise and/or connect to oneor more input/output (I/O) devices (e.g., via the communicationsinterface 138) including, but not necessarily limited to a display, amouse, a touchpad, a keyboard, and so on.

The communications interface 138 and/or the processor 134 can beconfigured to communicate with a variety of different networksincluding, but not necessarily limited to: a wide-area cellulartelephone network, such as a 3G cellular network, a 4G cellular network,or a global system for mobile communications (GSM) network; a wirelesscomputer communications network, such as a WiFi network (e.g., awireless local area network (WLAN) operated using IEEE 802.11 networkstandards); an internet; the Internet; a wide area network (WAN); alocal area network (LAN); a personal area network (PAN) (e.g., awireless personal area network (WPAN) operated using IEEE 802.15 networkstandards); a public telephone network; an extranet; an intranet; and soon. However, this list is provided by way of example only and is notmeant to be restrictive of the present disclosure. Further, thecommunications interface 138 can be configured to communicate with asingle network or multiple networks across different access points.

In one specific embodiment illustrated in FIGS. 1G through 1I, the fiberoptic sensor 100 can function as a fiber optic anemometer. In thisembodiment, the transmitted light 140 (e.g., white-light centered at1550 nm) is injected through the optical fiber 102 to the FP defined inthe fiber optic sensor 100 by the optical fiber 102 and the siliconlayer 108, and the reflection spectrum of the reflected light 162 can berecorded by a high-speed spectrometer 128. At the same time, heatinglight 142 from a heating light source 146 (e.g., 635 nm diode laser) canbe guided through the same optical fiber 102 to the heat the FP. Siliconhas a band gap energy of 1.11 eV and is highly transparent to thetransmitted light 140 but is opaque to the heating light 142. Therefore,the FP temperature can be effectively increased by the heating light142. When air moves (e.g., air convection 144) over the surface of a hotsilicon layer 108, a cooling effect from the moving air reduces thetemperature of the silicon layer 108 and the FP and introduces a shiftto the fringe valley wavelength of the reflection spectrum, asschematically shown in FIGS. 1H and 1I. The wavelength shift can beseparated by the spectrometer 128 and/or the controller 132 into awind-temperature-induced wavelength shift and a wind-speed-inducedwavelength shift. As a result, temperature self-compensated measurementof wind speed can be achieved by comparing the shift in the wavelengthsof a fringe valley when the heating laser is turned on and off todetermine temperature-compensated wind speed. It should be pointed outthat although it is implemented as an anemometer in this example, it isnot limited to measuring only the wind or air flow. Any other kind offlows (e.g., water flow) that can bring about the cooling effects to theheated sensor head can be measured.

Example Processes

The following discussion describes example techniques for utilizing afiber optic sensor and fiber optic sensing system, such as the fiberoptic sensor 100 and fiber optic sensing system 130 described in FIGS.1A through 1I. FIG. 2A depicts an example process 200 for fabricatingthe fiber optic sensor 100.

As shown in FIG. 2A, a light source is caused to transmit light througha fiber optic to a fiber optic sensor (Block 202). In thisimplementation, controller 132 can cause light source 126 to transmitlight (e.g., transmitted light 140) through optical fiber 102 andcirculator 124 to fiber optic sensor 100. Controller 132 can control theduration and intensity that the light source 126 transmits thetransmitted light 140. In some specific implementations, controller 132can cause heating light source 146 to transmit heating light 142 throughthe optical fiber 102 to the fiber optic sensor 100 and the siliconlayer 108 for providing heat.

Reflected light from the fiber optic sensor is received using aspectrometer (Block 204). The spectrometer 128 can receive the reflectedlight 162 and associated spectra, which can be recorded and/or analyzedby spectrometer 128 and/or controller 132.

An output from the spectrometer is analyzed based on the receivedreflected light (Block 206). In implementations, the controller 132and/or the spectrometer 128 can analyze the reflected light 162 and thespectra to determine a wavelength shift in the spectra, which indicatesa change in temperature. A variety of methods may be utilized to analyzeand/or determine the wavelength shift in the spectra and for trackingthe average wavelength. In a specific embodiment, analyzing an outputfrom the spectrometer based on received reflected light can includeusing an average wavelength tracking method to further increase theresolution of wavelength and/or measurand.

The following discussion describes example techniques for fabricating afiber optic sensor, such as the fiber optic sensor 100 described inFIGS. 1A through 1I. FIG. 2B depicts an example process 300 forfabricating the fiber optic sensor 100. FIGS. 3A through 3E illustrate asection an exemplary fiber optic sensor 100 during fabrication (such asthe fiber optic sensors 100 described in FIGS. 1A through 1I).

As shown in FIG. 2B, a silicon pillar is formed on a silicon substrate(Block 302). FIG. 3A illustrates forming at least one silicon pillar 150that will function as a sensor head for the fiber optic sensor 100. Inone specific implementation, a double-side-polished silicon wafer (e.g.,200 μm thick) can be bonded on top of another larger silicon wafer usinga layer of photoresist 154. The larger silicon wafer can function as asilicon substrate 148 to facilitate the fabrication and later as asupport for the fabricated silicon pillar 150. Then another layer ofphotoresist 154 can be coated on the top of the double-side-polishedsilicon wafer and patterned accordingly using photolithographytechniques. The patterned top silicon layer can be etched all the way tothe silicon substrate 148 and the second layer of photoresist 154 using,for example, deep-reactive-ion-etching, leaving the upstanding siliconpillar(s) 150 attached to the silicon substrate 148.

Then, a thin film adhesive is formed on a glass substrate (Block 304).In some specific embodiments, such as the one illustrated in FIG. 3B, anadhesive 156 including a thin film of UV-curable glue can be spin-coatedon a piece of glass substrate 158. It is contemplated that forming athin film adhesive 156 may include using other adhesives and/or othermethods for depositing and/or forming the adhesive 156. In embodiments,the adhesive 156 may include a thin film adhesive (e.g., UV-curableglue, an epoxy-based adhesive, and/or a gel-based adhesive).

Shown in FIG. 3C, an end face of an optical fiber is pressed onto thethin film adhesive (Block 306). In implementations, the adhesive 156 canbe transferred to a cleaved and cleaned end face 152 of an optical fiber102 by pressing the end face 152 of the optical fiber 102 to theadhesive 156 on the glass substrate 158. Subsequently, the end face 152and the adhesive 156 can be released from the glass substrate 158.

As illustrated in FIGS. 3D and 3E, the optical thin film adhesive on theend face is placed onto the silicon pillar to provide the fiber opticsensor (Block 308). The optical fiber 102 with the silicon pillar 150(silicon layer 108) attached can be lifted from the silicon substrate148 and the second layer of photoresist 154. Further, fabrication of thefiber optic sensor 100 may include cleaning residual photoresist 154from the end of the silicon pillar 150 (e.g., with alcohol). Due to theultra-thin thickness of the residual photoresist 154 between the opticalfiber 102 and the silicon pillar 150/silicon layer 108, the reflectionspectrum of the FP cavity within the fiber optic sensor 100 is notaffected.

Then, the thin film adhesive is cured (Block 310). In implementations,the adhesive 156 can be cured, for example, by UV irradiation (e.g., UVlight 160). It is contemplated that other bonding technology may beimplemented to mount the silicon layer 108 (or other material) to theend face 152 of the optical fiber 102, such as physical contact bonding.

It is contemplated that the above steps may be repeated to form a fiberoptic sensor 100 with cascaded Fabry-Pérot cavities. For example, theoptical fiber 102 with the silicon pillar 150 may be further pressedonto a second adhesive 166 on a glass substrate, placed on a secondsilicon pillar (e.g., second silicon layer 168), and cured using UVlight, to form a fiber optic sensor 100 with two Fabry-Pérotinterferometers. Further Fabry-Pérot interferometers may be fabricatedby repeating the above steps.

Conclusion

Although the subject matter has been described in language specific tostructural features and/or process operations, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

What is claimed is:
 1. A fiber optic sensor, comprising: an opticalfiber configured to be coupled to a light source and a spectrometer; anda first silicon layer disposed on an end face of the optical fiber,where the first silicon layer defines a Fabry-Pérot interferometer and asensor head, where the sensor head reflects light from the light sourceto the spectrometer, and where the first layer includes a doublesided-polished silicon pillar less than approximately 200 μm in length.2. The fiber optic sensor of claim 1, wherein the optical fiber includesa fused silica single mode fiber.
 3. The fiber optic sensor of claim 1,wherein the optical fiber includes a circulator.
 4. The fiber opticsensor of claim 1, wherein the first silicon layer has a diameterapproximately the same as a diameter of the optical fiber.
 5. The fiberoptic sensor of claim 1, further comprising: the light source coupled tothe optical fiber; the spectrometer coupled to the optical fiber; and acontroller coupled to the spectrometer.
 6. The fiber optic sensor ofclaim 5, wherein the spectrometer is based on a transmission grating anda diode array operating in the 1550 nm wavelength window.
 7. The fiberoptic sensor of claim 1, further comprising: a heating light sourcecoupled to the optical fiber.
 8. The fiber optic sensor of claim 7,wherein the heating light source includes a short wavelength diodelaser.
 9. A process for fabricating a fiber optic sensor, comprising:forming at least one silicon pillar on a silicon substrate; forming athin film adhesive on a glass substrate; pressing an end face of anoptical fiber onto the thin film adhesive and releasing; placing thethin film adhesive on the end face onto the at least one silicon pillar;and curing the thin film adhesive to form the fiber optic sensor. 10.The process for fabricating the fiber optic sensor in claim 9, whereinthe at least one silicon pillar is approximately 80 μm in diameter andapproximately 200 μm in length.
 11. The process for fabricating thefiber optic sensor in claim 9, wherein the at least one silicon pillarincludes a metal coating disposed on an end face of the at least onesilicon pillar that is distal from the optical fiber.
 12. The processfor fabricating the fiber optic sensor in claim 9, wherein the thin filmadhesive includes at least one of a UV-curable glue, an epoxy-basedadhesive, or a gel-based adhesive.
 13. A fiber optic sensor, comprising:an optical fiber configured to be coupled to a light source and aspectrometer; and a first silicon layer disposed on an end face of theoptical fiber, where the first silicon layer defines a Fabry-Pérotinterferometer and a sensor head, where the sensor head reflects lightfrom the light source to the spectrometer, wherein the first siliconlayer includes a double sided-polished silicon pillar with a diameterless than approximately 100 μm.
 14. A fiber optic sensor, comprising: anoptical fiber configured to be coupled to a light source and aspectrometer; a first silicon layer disposed on an end face of theoptical fiber, where the first silicon layer defines a Fabry-Pérotinterferometer and a sensor head, where the sensor head reflects lightfrom the light source to the spectrometer; and a second silicon layerdisposed on the first silicon layer, where a first silicon layer lengthis different from a second silicon layer length.